fix($http): fix response headers with an empty value

Empty response header values are legal (http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html).
Return an empty string instead of null, which is returned when the header does not exist.

Closes angular#7779

Author: jamshid

src/ng/http.js

@@ -63,7 +63,8 @@ function headersGetter(headers) {
     if (!headersObj) headersObj =  parseHeaders(headers);
 
     if (name) {
-      return headersObj[lowercase(name)] || null;
+      name = lowercase(name);
+      return headersObj.hasOwnProperty(name) ? headersObj[name] : null;
     }
 
     return headersObj;

test/ng/httpSpec.js

@@ -816,6 +816,15 @@ describe('$http', function() {
       });
 
 
+      it('should handle empty response header', function() {
+       $httpBackend.expect('GET', '/url', undefined)
+           .respond(200, '', { 'Custom-Empty-Response-Header': '' });
+       $http.get('/url').success(callback);
+       $httpBackend.flush();
+       expect(callback).toHaveBeenCalledOnce();
+       expect(callback.mostRecentCall.args[2]('custom-empty-response-Header')).toBe('');
+     });
+
       it('should have delete()', function() {
         $httpBackend.expect('DELETE', '/url').respond('');
         $http['delete']('/url');